Question

I cannot get the exact difference between “nscanned” and “nscannedObjects” in the Mongodb’s explain query output.

On MongoDB Explain documentation I can read:

nscanned Number of items (documents or index entries) examined. Items
might be objects or index keys. If a “covered index” is involved,
nscanned may be higher than nscannedObjects.

nscannedObjects Number of documents scanned.

What’s the different between these two fields?
And more specific what does exactly mean when I have a query, which uses a BtreeCursor (an index), and these two fields have two different values, for example:

{
    "cursor" : "BtreeCursor a_1_b_1",
    "isMultiKey" : false,
    "n" : 5,
    "nscannedObjects" : 5,
    "nscanned" : 9, 
    (...)
}

I know what a “covered index” is.
I would like to understand exactly what the query did in the example above.
Did it pass through (“scanned”) 9 elements (nscanned = 9), where all of them are index entries and read (“examined”) the value of only 5 of them (nscannedObjects = 5) to produce the result set?

Answer 1

This means that :
The query returned 5 documents – n
scanned 9 documents from the index – nscanned
and then read 5 full documents from the collection – nscannedObjects

Similar example is given at :
http://docs.mongodb.org/manual/reference/method/cursor.explain/#cursor.explain